When I did my fire dye tele I used black. It makes a difference when you look at the dyes right next to each other. The dark brown looks very black on it's own, but the whole thing will have a warmer look to it if you use it. The black tends to add a blue-ish hue. Once again, it is splitting hairs. But if you are going to do the effort, might as well get the brown.
I used the Reranch alcohol dyes. So my experience is slanted according to that product. I found you could put a thin line of dark around the edge with a q-tip and blend it into the rest with a paper towel that had acetone on it. Acetone blended things nicely. I also found that when you think a touch more blending might make it look better, that is your cue to quit. It generally doesn't, and you lose the striking nature of the dark sanded back areas. Finally, before you clear coat it, use sanding sealer. The dyes can migrate into the clear and make it look much more yellow. Fine for somethings, but I wouldn't use that approach on a finish like this. The clear being clear makes it look like you can swim in it. Yellow, not so much.
Patrick